There is growing evidence that plasminogen activator is involved in processes which require extracellular proteolysis, including normal physiological events and neoplastic disease. It is known that this enzyme is produced by the epithelial cells of the rat ventral prostate but its function is obscure. Exploration of the hormonal control of plasminogen activator in the rat ventral prostate in vivo and in vitro could help clarify the role of this enzyme in prostatic secretions, and might also shed light on the hormonal maintenance of the prostate. Trophoblast cells in the early mouse embryo also synthesize and secrete plasminogen activator. Examination of the control of enzyme production in cultures derived from mouse blastocysts might identify molecules which are affecting early embryogenesis. In addition, comparison of normal trophoblast with neoplastic trophoblast derived from choriocarcinoma may help explain the pathology of this disease.